The field of the invention is magnetic resonance angiography ("MRA"), and particularly, dynamic studies of the human vasculature using contrast agents which enhance the NMR signals.
Diagnostic studies of the human vasculature have many medical applications. X-ray imaging methods such as digital subtraction angiography ("DSA") have found wide use in the visualization of the cardiovascular system, including the heart and associated blood vessels. Images showing the circulation of blood in the arteries and veins of the kidneys and the carotid arteries and veins of the neck and head have immense diagnostic utility. Unfortunately, however, these x-ray methods subject the patient to potentially harmful ionizing radiation and often require the use of an invasive catheter to inject a contrast agent into the vasculature to be imaged.
Magnetic resonance angiography (MRA) uses the nuclear magnetic resonance (NMR) phenomenon to produce images of the human vasculature. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. Each measurement is referred to in the art as a "view" and the number of views determines the resolution of the image. The resulting set of received NMR signals, or views, are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. The total scan time is determined in part by the number of measurement cycles, or views, that are acquired for an image, and therefore, scan time can be reduced at the expense of image resolution by reducing the number of acquired views.
MR angiography (MRA) has been an active area of research. Two basic techniques have been proposed and evaluated. The first class, time-of-flight (TOF) techniques, consists of methods which use the motion of the blood relative to the surrounding tissue. The most common approach is to exploit the differences in signal saturation that exist between flowing blood and stationary tissue. The improvement in blood-tissue contrast is due to the stationary tissues experiencing many excitation pulses and becoming saturated. Flowing blood, which is moving through the excited section, is continually refreshed by spins experiencing fewer excitation pulses and is, therefore, less saturated. The result is the desired image contrast between the high-signal blood and the low-signal stationary tissues.
To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. As described in U.S. Pat. No. 5,417,213 the trick is to acquire the central k-space views at the moment the bolus of contrast agent is flowing through the vasculature of interest. This is not an easy timing to achieve as part of a routine clinical procedure because the delay time between intravenous injection to arrival in the arterial vasculature of interest is highly patient-dependent. Therefore, some means is required for determining this delay time and synchronizing MR data acquisition to the contrast bolus profile. Such synchronization is necessary to provide adequate vessel contrast and to prevent artifacts such as edge enhancement of the vessel. Various means have been developed to provide accurate timing including: use of a small test injection of contrast as described by J. K. Kim, R. I. Farb, and G. A. Wright, Test Bolus Examination in the Carotid Artery at Dynamic Gadolinium-enhanced MR Angiography, Radiology, 1998, 206:283-289; real-time line scanning described by T. K. Foo, S. Manojkumar, M. R. Prince, and T. L. Chenevert, Automated Detection of Bolus Arrival and Initiation of Data Acquisition in Fast, Three-dimensional, Gadolinium-enhanced MR Angiography, Radiology 1997, 205:137-146; and real-time fluoroscopic imaging as described by A. H. Wilman, S. J. Riederer, B. R. King, J. P. Debbins, P. J. Rossman, R. L. Ehman, Fluoroscopically Triggered Contrast-Enhanced Three-dimensional MR Angiography with Elliptical Centric View Order: Application to the Renal Arteries", Radiology 1997, 205:137-146.
The in vivo contrast enhancement profile provided by the passage of a contrast agent bolus closely matches a gamma-variate function as described by the general equation: EQU C(t)=Ate.sup.-.zeta.t.
As shown in FIG. 4, as a result of the contrast agent passage the acquired NMR signal is enhanced considerably for a short time interval and then the enhancement tapers off. Consequently, even if the MR acquisition is accurately synchronized to the contrast bolus, only a small number of views (usually the central k-space views) are acquired while the T1 shortening associated with high contrast agent concentration is at its peak. The bulk of the image is acquired while contrast concentration is decreasing and T1 time is increasing.
The predominant method for acquiring MRA data is to detect the arrival of the contrast bolus in the region of interest and trigger a centric view order image acquisition. As described, for example, in U.S. Pat. No. 5,122,747 the views are arranged in an order which samples the central region of k-space first and the most peripheral regions last. As shown in FIG. 4, if the contrast bolus arrives very quickly in the region of interest as indicated by curve 10, the leading edge of the contrast profile is very steep and the central k-space views acquired at the beginning of the triggered scan are synchronized very well with the peak contrast enhancement. High quality arterial phase MRA images are routinely obtained in this situation. On the other hand, when the bolus has a longer arrival time the leading edge of the contrast profile is not as steep as indicated by curve 12, and the initial views acquired after detection of bolus arrival will occur before peak contrast enhancement occurs. If a centric view order is employed in this latter situation, slight, but noticeable and undesirable edge enhancement occurs in the reconstructed image because some peripheral k-space views are acquired with greater contrast enhancement than some of the central k-space views.
One solution to this problem is to delay the centric view order scan several seconds after the arrival of the contrast bolus is detected. While this ensures that the central k-space views are acquired at peak contrast, it wastes a significant amount of time before the scan is triggered during which the contrast is relatively high. As a result, some of the peripheral views acquired at the end of the scan have much reduced contrast enhancement and image SNR is adversely affected.